In Drosophila melanogaster there exists a class of mutations called "Minutes," which is characterized by phenotypic abnormalities caused by defective protein synthesis. While several of these loci have been identified as encoding ribosomal proteins, others are likely to encode other components of the protein synthesizing apparatus. The Minute(1)1B [M(1)1B] gene is likely a member of the latter category, in that its putative polypeptide product has a molecular weight of approximately 128 KD, much larger than the largest ribosomal protein (approximately 50 KD). From cloning and sequencing the M(1)1B gene, we have identified an open reading frame that could encode an 1133 amino acid polypeptide. The protein has no homology to any known protein sequences or to proteins that could be produced by the GenEMBL nucleic acid data base. Cellular fractionation and immunological techniques utilizing polyclonal antisera made against three nonoverlapping regions of the open reading frame have localized the protein in a subregion of the nucleus, possibly the nucleolus. This suggests that the protein may have some function in ribosome biogenesis. The protein is produced at high levels during stages of development when large numbers of ribosomes are produced.